Oxidative carbonylation of alcohols to dialkyl oxalates using a heterogeneous iron promoted Pd-V-P catalyst

ABSTRACT

Synthesis of dialkyl oxalates by the heterogeneous catalyzed oxidative carbonylation of liquid monohydric saturated alcohols of from 1 to 20 carbon atoms with carbon monoxide and oxygen or an oxygen-containing gas in the presence of a catalytic amount of a catalyst comprising palladium or a salt thereof in combination with a crystalline vanadium-phosphorus-iron containing compound.

BACKGROUND OF THE INVENTION

The preparation of dialkyl oxalate esters by the homogeneous catalyzedoxidative carbonylation of alcohols in the presence of metal saltcatalysts, redox agents, dehydrating agents and other compounds such asamines, carbonates, nitrates, hydroxides and ureas is well known. Anarticle by Donald M. Fenton and Paul J. Steinwand, Journ. of Org. Chem.,Vol. 39, Nos. 5, 1974, pp. 701-704 describes a general mechanism for theoxidative carbonylation of alcohols to yield dialkyl oxalates using apalladium redox system, oxygen and dehydrating agents. Typical prior artpatents disclosing homogeneous catalyzed oxidative carbonylation ofalcohols to prepare oxalate esters are U.S. Pat. Nos. 3,393,136;3,994,960; 4,005,129; 4,005,130; 4,076,949; 4,118,589 and 4,281,174 aswell as West German Patent No. 2,213,435 and West GermanOffenlegungschrift No. 2,601,139.

The present invention provides an improved process of the production ofdialkyl oxalates by employing an insoluble easily recoverabletetra-metallic containing heterogeneous catalyst for the oxidativecarbonylation of alcohols.

U.S. Pat. No. 4,229,591 describes a process for the preparation of adiester of oxalic acid by contacting an ester of nitrous acid or analcohol and a nitrogen oxide or hydrate thereof in the gaseous phase inthe presence of a solid catalyst containing palladium or a salt thereofsuch as palladium on activated carbon.

Japanese Kokai No. 75-157,311 discloses the preparation of oxalic acidesters by reacting a C₁ to C₂₀ monohydric alcohol, carbon monoxide andmolecular oxygen in the presence of a supported Group VIII metal and aGroup IB, IIB, III, IV, V, VI, and VIIIB metal, aluminum, iron, cobaltor nickel.

U.S. Pat. No. 4,039,572 discloses the preparation of diesters ofdicarboxylic acids by the oxidative carbonylation of olefins andalcohols using a carrier supported catalyst consisting of (1) a platinumgroup metal compound and (2) a compound of a metal having an atomicnumber of not less than 22 which has been reduced to a metal and has aratio of (2) to (1) of from 0.0005:1 to 10:1 gram atoms.

The oxalate products of this invention have many commercial applicationsand are used as solvents, dye intermediates, for the preparation ofpharmaceuticals as well as feedstock for hydrogenation to ethyleneglycol by, for example, the process described in U.S. Pat. No.4,112,245.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a process forthe synthesis of dialkyl oxalates by the liquid or vapor phase oxidativecarbonylation of a liquid alcohol with a mixture of carbon monoxide andoxygen in the presence of a catalytic amount of an insolubletetra-metallic containing heterogeneous catalyst comprising acombination of (1) palladium metal or a salt thereof and (2) acrystalline vanadium oxide-phosphorous oxide-iron oxide containingcompound. Good yield and selectivity of dialkyl oxalates is obtainedespecially with the lower alcohols. In addition, the Pd-V-P-Fe metalcontaining catalyst may be supported on, for example silica (SiO₂) toprovide still greater selectivity and catalyst productivity.

It is a primary object of this invention to provide a process for thepreparation of dialkyl oxalates employing a Pd-V-P-Fe containingheterogeneous catalyst system.

It is a further object of this invention to provide a specifcheterogeneous catalytic mechanism for the employment of palladium orsalts thereof with a crystalline compound of oxides of vanadium,phosphorus and iron in an oxidative carbonylation process employingalcohol, carbon monoxide and oxygen as reactants.

These and other objects and advantages of this invention will becomeapparent from the description of the invention which follows and fromthe claims.

DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that good yields ofdialkyl oxalates at improved selectivities and catalyst productivity canbe obtained from the reaction of an alcohol and a mixture of carbonmonoxide, oxygen or an oxygen containing gas, such as air, at elevatedtemperatures and pressures in the presence of a solid catalytic mixtureof palladium or a salt thereof, in combination with a crystallinecompound consisting of vanadium in the form of its oxide, an oxide ofphosphorus and an iron oxide which may be in the form of a single phasecrystalline V-P-Fe compound or an intimately combined single phasecrystalline vanadium oxide-phosphorus oxide compound and a crystallineiron oxide. The Pd-V-P-Fe containing catalyst is in the heterogeneousstate in the reaction at reaction conditions and while vapor phasereactions employing a bed of catalyst may be employed the reactions aregenerally carried out with a slurry of the catalyst mixture, supportedor unsupported, in the reactant alcohol. The catalyst may be on inertsupport materials such as alumina, silica gel, alumino-silicates,activated carbon or zeolites, and when so employed will generallyprovide a higher percent selectivity of product and greater catalystproductivity.

The reaction between the alcohol, carbon monoxide, and oxygen may becarried out in an autoclave or any other high pressure reactor. Althoughthe order of addition of reactants and the heterogeneous Pd-V-P-Fecontaining catalyst may vary, a general procedure is to charge thealcohol and catalyst (supported or unsupported) into the reactionvessel, and then introduce the proper amount of carbon monoxide andoxygen to the desired reaction pressure and then heat the mixture to thedesired temperature for the appropriate period. The reaction can becarried out batchwise or as a continuous process and the order ofaddition of the reactants may also be varied to suit the particularapparatus employed. The addition of the oxygen or oxygen-containing gas,such as air, can be a pulsed or continuous addition to the reactionsystem. The reaction products are recovered and treated by anyconventional method such as distillation and/or filtration, etc. toeffect separation of the oxalate ester from unreacted materials,catalyst, by-products, etc.

The alcohols which may be employed in concentrations of from about 50 to99.7 weight percent, preferably 77 to 94 weight percent and suitable foruse in the process of the present invention can be monohydric saturatedaliphatic and alicyclic alcohols and may contain other substitutentssuch as amido, alkoxy, amino, carboxy, cyano, etc. radicals in additionto the hydroxyl group. The substituents, in general, do not interferewith the reaction.

The alcohols which are employed may be primary, secondary, or tertiaryalcohols and conform to the general formula ROH, wherein R is anoptionally substituted aliphatic, or alicyclic group containing from 1to 20 carbon atoms and preferably unsubstituted aliphatic alcoholscontaining from 1 to 10 carbon atoms and more preferably 1 to 4 carbonatoms. In general, the alcohol is one which is normally liquid under theconditions employed in the carboxylation reaction. Representativealcohols especially suitable for use in this invention are saturatedmonohydric alcohols such as methyl, ethyl, n-, iso-, sec-, andtert-butyl, amyl, hexyl, octyl, lauryl, n- and iso-propyl, cetyl,benzyl, chlorobenzyl and methoxy-benzyl alcohols as well as for example,cyclohexanol, heptanols, decanols, undecanols, 2-ethyl hexanol, nonanol,myristyl alcohol, stearyl alcohol, methyl cyclohexanol, pentadecanol,oleyl and eicosonyl alcohols, and the like. The preferred alcohols arethe primary and secondary monohydric saturated aliphatic alcohols, suchas methanol, ethanol, 1- and 2-propanol, n-butyl alcohol etc., up to 10carbon atoms.

The palladium salts which may be employed in the process of thisinvention and in forming the catalyst mixture include the palladium (II)compounds or mixtures thereof. Among the chemical forms of the palladiumcompounds which can be used as such or as mixtures are the palladium,halides, sulfates, carboxylates, acetates, oxides, and nitrates,preferably the palladium (II) halides. Representative palladium saltcompounds include, for example palladium (II) oxide, palladium (II)chloride, palladium (II) sulfate, palladium (II) acetate, palladium (II)iodide, palladium (II) oxalate, palladium (II) propionate, etc. Thepalladium content of the catalyst system may range from about 0.25 to 5weight percent on the crystalline vanadium oxide-phosphorus oxide-ironoxide containing compound.

The vanadium employed in the preparation of the crystalline catalystmixture is in the essentially insoluble oxide form and may be V₂ O₅, V₂O₄ or V₂ O₃ or mixtures thereof as well as pentavalent salts such asammonium metavanadate.

The oxides of phosphorus employed in the process of this invention andin forming the catalyst mixture include phosphoric acid, phosphorusacid, phosphorus trioxide (or phosphorus anhydride) P₄ O₆, phosphoruspentoxide (or phosphoric anhydride) P₂ O₅ as well as oxides ofphosphorus which may be in the form of P₄ O₇, P₄ O₈ or P₄ O₉ orcombinations thereof, all of which are generally produced by the directoxidation of phosphorus. Any combination of a single phase crystallinevanadium-phosphorus compound may be employed in the catalyst systemalong with the iron oxide and may exist for example as single phasecrystalline vanadium(IV)bis(metaphosphate), VO(PO₃)₂ type compoundcombined with a crystalline iron oxide or the mixed V-P oxide catalystsof U.S. Pat. No. 4,333,853 combined with iron oxide.

The iron employed in the process of this invention as well as to formthe iron promoted catalyst system are the oxides of iron and includeFeO, Fe₂ O₃, and Fe₃ O₄ or mixtures thereof. Iron salts such as ferricoxalate, ferric and ferrous sulfate, ferric bromide, ferrous acetate,and ferric acetylacetonate, etc. may be used in preparation of thecatalyst provided they are converted to an iron oxide form undercalcining conditions of catalyst preparation to the Pd-V-P-Fecombination catalyst.

While a method for the preparation a the Pd-V-P-Fe supported orunsupported catalyst is set forth hereinafter in the examples, any otherknown method for the preparation of any such catalyst mixture may beused as long as it produces a Pd-V-P-Fe heterogeneous crystalline typecatalyst having an appropriate ratio of one metal to the other in theoxidative carbonylation catalyst system. The atomic ratio of the Pd to Vto P to Fe as metals in the unsupported catalyst employed may range fromabout 0.003:1:0.5:0.1 to about 0.32:1:5:5 and generally ranges from0.03:1:1:0.5 to 0.15:1:3:2 with an optimal atomic ratio of 0.09:1:1.2:1.A supported catalyst, for example on SiO₂, will generally have an atomicratio of Pd:V:P:Fe as the metals ranging from 0.01:1:0.5:0.1 to0.67:1:5:5 and preferably 0.06:1:1:0.5 to 0.32:1:3:2 with an optimalatomic ratio of 0.19:1:1.2:1. The reaction is generally carried out inthe presence of a catalytic proportion of the heterogeneous catalystcombination and will proceed with small amounts of the catalysthereinabove described. Generally the proportions of catalyst used in thereaction as a slurry mixture with alcohol will be equivalent to betweenabout 0.05 to 5 weight percent based on the alcohol employed and ispreferably employed in amounts of between 0.5 to 2.5 percent by weightof the alcohol employed.

Although not required, solvents, if desired, which are chemically inertto the components of the reaction system may be employed. Suitablesolvents include, for example, hydrocarbons such as hexane, heptane,toluene and xylene; ethers such as tetrahydrofuran, diethylether,diphenylether; halogenated hydrocarbons such as methylene chloride,chlorobenzene and dichlorobenzene; organic esters such as ethyl acetate,n-propyl formate, isopropyl acetate, sec- and iso-butyl acetate, amylacetate, cyclohexyl acetate, n-propyl benzoate; lower alkyl phthalates,etc. and the alkyl sulfones and sulfoxides such as propylene ethylsulfoxide, diisopropyl sulfone, diisooctyl sulfoxide, acetone,cyclohexanone, etc.

As indicated above the reaction can be suitably performed by introducingthe oxygen and carbon monoxide at a desired pressure into contact withthe alcohol/catalyst mixture comprising the palladium-vanadiumoxide-phosphorus oxide-iron oxide catalyst either supported orunsupported and heating to the desired temperature. In general, a carbonmonoxide pressure of about 400 psig to about 5000 psig partial pressureand preferably from 900 psig to about 2200 psig is employed.Stoichiometric quantities of carbon monoxide are generally employed.However, an excess of carbon monoxide may be employed, for example, incontinuous processes where a large excess of or high carbon monoxiderequirements are generally utilized, a suitable recycle of the unreactedcarbon monoxide may be employed. The reaction will proceed attemperatures of from about 40° C. to 150° C. It is generally preferredto operate the process at temperatures in the range of 75° C. to 120° C.to obtain a convenient rate of reaction. Heating and/or cooling meansmay be employed interior and/or exterior of the reaction to maintain thetemperature within the desired range.

At least stoichiometric amounts of oxygen or an oxygen-containing gassuch as air may be employed and at any oxygen partial pressure such thatthe explosive range is avoided. Thus, the concentrations of oxygenshould be low enough so that the reaction mixture is not potentiallyexplosive. The Handbook of Chemistry and Physics, 48th Edition, 1967indicates that the explosive limits of pure oxygen in carbon monoxide is6.1 to 84.5 volume percent and air in carbon monoxide to be 25.8 to 87.5volume percent. The volume percent of the oxygen in the oxygen-carbonmonoxide mixture usually amounts to about 3 to 6 percent. In carryingout the reaction the oxygen is charged to the reaction vessel to thedesired pressure and concentration and may be charged in portions forsafety reasons. Total carbon monoxide-oxygen pressures will rangebetween about 500 psig and 6000 psig.

The reaction time is generally dependent upon the alcohol being reacted,temperature, pressure and on the amount of the catalyst mixture beingcharged as well as the type of equipment being employed. Reaction timeswill vary dependent on whether the process is continuous or batch butwill generally run for from 0.5 to 2 hours under batch conditions. Thereaction is limited by the available oxygen, alcohol and carbonmonoxide.

The following examples are provided to illustrate the invention inaccordance with the principles of this invention but are not to beconstrued as limiting the invention in any way except as indicated bythe appended claims.

EXAMPLE 1 Preparation of a Pd-V-P-Fe Containing Catalyst

A 1 liter, three necked flask equipped with a mechanical stirrer, acondenser with a Dean-Stark water trap, a thermometer and an additionfunnel was charged with 30 g. of vanadium pentoxide, 26.34 g. iron (III)oxide (Fe₂ O₃), 320 ml of isobutyl alcohol and 160 ml of benzyl alcohol.The resulting heterogeneous mixture was stirred at reflux for 4 hoursduring which time 2 ml of water was collected in the trap. The suspendedsolids underwent a color change from brown to black indicative of aV^(V) to V^(IV) reduction. The slurry was then cooled to roomtemperature and a solution of 45.64 g. of 85 percent ortho-phosphoricacid in 80 ml of isobutanol added slowly over a period of 45 minutes viathe addition funnel. A slight exotherm occurred. The resulting mixturewas stirred at reflux for 8 hours over which time 10 ml of water wasremoved. The suspended solids underwent a color change of from black toblue 30 minutes after completion of the phosphoric acid addition. Aftercooling to room temperature, the solids were collected on a Buchnerfunnel, washed with 100 ml of isobutanol and air dried for 3 hours.Residual solvents were removed by heating in a vacuum oven at 100° C.for 5 hours. The resulting hard, reddish solids were ground to a powderand calcined from 100° C. to 400° C. at a rate of 1° C. per minute andthen held at 400° C. for 16 hours. The weight of the calcined reddishcrystalline solids equaled 80 g. 19.40 g. of the calcinedvanadium-phosphorus-iron oxides as a solid material was added to a 35 mlsolution of 1 N ammonium hydroxide containing 0.999 g. of palladiumchloride dissolved therein. The resulting slurry was stirred for 1 hourat room temperature and then heated on a steam bath with stirring tonear dryness. After drying at 120° C. for 2 hours, the catalyst wascalcined from 100° C. to 400° C. at a rate of 1° C. per minute and thenheld at 400° C. for 16 hours. The resulting catalyst weight equaled19.90 g. The atomic ratio of palladium to vanadium to phosphorus to ironas metal in the catalyst was 0.09:1:1.2:1.

A SiO₂ supported Pd-V-P-Fe containing catalyst was prepared inaccordance with the above procedure by adding 110 g. of silica gel tothe vanadium pentoxide-iron (III) oxide-isobutanol-benzyl alcoholmixture, prior to reflux and addition of the orthophosphoric acid, togive a catalyst containing a palladium to vanadium to phosphorus to ironatomic ratio of 0.19:1:1.2:1 on silica.

EXAMPLE 2 (Comparative)

To a 500 cc stainless steel stirred autoclave was charged 200 g. ofmethanol and 2.5 g. of a palladium-vanadium oxide catalyst (1 percent Pdand 56 percent V as the metal). The autoclave was brought to atemperature of 100° C. and 1200 psig of carbon monoxide added withstirring. 400 psig air was charged and then 900 psig carbon monoxide tobring the total pressure to 2500 psig. The reaction was carried out for1 hour after which the reactor was cooled to ambient temperature andvented to ambient pressure and gas samples obtained. Solids wereseparated from liquid products by vacuum filtration. The liquid productwas analyzed by gas-liquid chromatography (glc) and titration methodsand the gaseous product was analyzed by gas chromatograph. Analysis ofthe products showed 4.86 mmole dimethyl oxalate, 12.6 mmole methylformate and 0.6 mmole carbon dioxide. Selectivity to dimethyl oxalatewas 42.4 percent with a catalyst productivity of 0.573 g/g-hr.

EXAMPLE 3 (Comparative)

The procedure of Example 2 was repeated using 2.5 g. of 2 weight percentpalladium metal on silica as catalyst. The reaction was carried out for1 hour under the same temperature and pressure conditions of Example 2.Analysis of the reaction products showed a selectivity to dimethyloxalate of 35.7 mole percent with a catalyst productivity of 0.18g/g-hr.

EXAMPLE 4

To a 500 cc stainless steel stirred autoclave equipped with an internalcooling coil was added a slurry of 200 g. of methanol and 2.5 g. ofsilica supported Pd-V-P-Fe containing catalyst with a Pd:V:P:Fe atomicratio of 0.19:1:1.2:1 as prepared by Example 1. While heating theautoclave to 100° C., 1200 psig carbon monoxide followed by 400 psig airand another 900 psig carbon monoxide was added with stirring to bringthe total pressure to 2500 psig. The reaction was carried out for 15minutes after which the reactor was cooled by pumping isopropanol cooledin dry ice through the internal coils. A temperature of -10°C. wasobtained in 30 minutes. The autoclave was vented to ambient pressure andgas samples obtained. The liquid product was separated from solids byvacuum filtration. The liquid product was analyzed by gas-liquidchromatography (glc) and titration methods and the gaseous product wasanalyzed by gas chromatograph. Analysis of the products showed 5.9 mmoledimethyl oxalate, 0.7 mmole oxalic acid and 0.3 mmole carbon dioxide.Selectivity to dimethyl oxalate was 96 mole percent with a catalystproductivity of 1.19 g/g-hr.

EXAMPLE 5

The procedure and operating conditions of Example 4 was repeated exceptthat 2.5 g. of unsupported Pd-V-P-Fe catalyst as prepared by Example 1and having a Pd:V:P:Fe atomic ratio of 0.09:1:1.2:1 was employed.Analysis of the reaction products showed 5.4 mmole dimethyl oxalate, 0.6mmole oxalic acid and 0.4 mmole carbon dioxide. Selectivity to dimethyloxalate was 94 mole percent with a catalyst productivity of 1.13 g/g-hr.

EXAMPLES 6 to 20

In Examples 6 to 20 which follow in table form (Tables 1 and 2), theprocedure of Example 4 was repeated using, 2.5 g. of an unsupportedpalladium-vanadium-phosphorus-iron containing catalyst with 3 weightpercent palladium and an atomic ratio of V:P:Fe of 1:1.2:1, 200 gms. ofvarious alcohol reactants and varying conditions with a 15 minutereaction time. Products were analyzed by gas liquid chromatography andtitration methods to give mole percent selectivity to the dialkyloxalate and catalyst productivity in gram/gram-hour.

                                      TABLE 1                                     __________________________________________________________________________    Example No.                                                                              6    7    8    9    10   11   12   13                              __________________________________________________________________________    Alcohol    MeOH.sup.(1)                                                                       MeOH MeOH MeOH MeOH MeOH EtOH.sup.(2)                                                                       EtOH                            Pressure (psig)                                                               Air        400  400  160  240  320  480  400  400                             CO         2100 2100 840  1260 1680 2520 2100 2100                            Temp. (°C.)                                                                       75   125  100  100  100  100  75   125                             Products (mmoles)                                                             Dialkyl Oxalate                                                                          2.1  5.2  2.4  3.4  4.2  7.3  1.5  1.2                             Oxalic Acid                                                                              0.2  0.5  0.2  0.4  0.4  0.8  0.1  0.1                             CO.sub.2   0.4  0.8  0.3  0.3  0.4  0.5  0.1  0.9                             Selectivity (Mole %)                                                                     85   67   90   93   92   85   94   59                              Catalyst Productivity                                                                    0.43 1.08 0.49 0.72 0.87 1.54 0.38 0.31                            (g/g-hr.)                                                                     __________________________________________________________________________     .sup.(1) MeOH = Methyl Alcohol                                                .sup.(2) EtOH = Ethyl Alcohol                                            

                                      TABLE 2                                     __________________________________________________________________________    Example No.                                                                              4    15   16   17   18   19   20                                   __________________________________________________________________________    Alcohol    EtOH PrOH.sup.(3)                                                                       BuOH.sup.(4)                                                                       BuOH BuOH BuOH BuOH                                 Pressure (psig)                                                               Air        400  400  400  320  480  400  400                                  CO         2100 2100 2100 1680 2520 2100 2100                                 Temp. (°C.)                                                                       100  100  100  100  100  75   125                                  Products (mmoles)                                                             Dialkyl Oxalate                                                                          4.0  2.0  0.5  0.6  1.5  0.7  1.2                                  Oxalic Acid                                                                              0.4  0.2  0.0  0.0  0.1  0.0  0.1                                  CO.sub.2   0.4  0.6  0.8  0.1  0.1  0.3  1.0                                  Selectivity (Mole %)                                                                     92   79   22   27   47   58   36                                   Catalyst Productivity                                                                    1.03 0.62 0.17 0.20 0.51 0.24 0.41                                 (g/g-hr.)                                                                     __________________________________________________________________________     .sup.(3) PrOH = nPropyl Alcohol                                               .sup.(4) BuOH = nButyl Alcohol                                           

We claim:
 1. A process for the preparation of dialkyl oxalates by theoxidative carbonylation of a liquid saturated monohydric aliphatic oralicyclic alcohol containing from 1 to 20 carbon atoms with a mixture ofcarbon monoxide and oxygen or an oxygen-containing gas at a temperatureof from about 40° C. to 150° C. and a total pressure of between about500 psig and 6000 psig in the presence of a catalytic amount of anessentially insoluble tetra-metallic-containing heterogeneous catalystcomprising palladium or a salt thereof in combination with a crystallinevanadium oxide-phosphorus oxide-iron oxide containing compound saidcatalyst containing as the metals an atomic ratio of palladium tovanadium to phosphorus to iron of from about 0.003:1:0.5:0.1 to about0.32:1:5:5 and recovering the desired dialkyl oxalate.
 2. A processaccording to claim 1 wherein the alcohol is a monohydric aliphaticalcohol containing from 1 to 10 carbon atoms.
 3. A process according toclaim 2 wherein the alcohol is selected from the group consisting ofmethanol, ethanol, propanol and butanol.
 4. A process according to claim3 wherein the alcohol is methanol.
 5. A process according to claim 1wherein the ratio is 0.09:1:1.2:1.
 6. A process according to claim 1wherein the vanadium-phosphorus-iron containing compound has an atomicratio of V:P:Fe of 1:1.2:1.
 7. A process according to claim 1 whereinthe Pd-V-P-Fe catalyst is employed in amounts of from about 0.05 to 5weight percent based on the alcohol employed.
 8. A process according toclaim 7 wherein between 0.5 to 2.5 weight percent catalyst is employed.9. A process according to claim 1 wherein the temperature is betweenabout 75° C. and 120° C. and carbon monoxide partial pressure is between900 psig and 2200 psig.
 10. A process according to claim 1 wherein thereaction is carried out in a solvent inert to the components of thereaction system.
 11. A process according to claim 1 wherein thePd-V-P-Fe containing catalyst is supported.
 12. A process according toclaim 11 wherein the support is SiO₂.
 13. A process for the preparationof dimethyl oxalate by the oxidative carbonylation of methanol withcarbon monoxide and oxygen or an oxygen-containing gas at a temperatureof from 75° C. to 120° C. and a carbon monoxide pressure of 2100 psigand oxygen or oxygen-containing gas pressure of 400 psig in the presenceof a catalytic amount of a tetra-metallic-containing catalyst comprisingpalladium or a salt thereof combined with a crystalline vanadiumoxide-phosphorus oxide-iron oxide containing compound said catalystbeing present in the heterogeneous phase and containing as the metals anatomic ratio of palladium to vanadium to phosphorus to iron of fromabout 0.003:1:0.5:0.1 to about 0.32:1:5:5.